Huge skulls clues to snake evolution

Yurlunggur would have looked like this, 6 metres long and with a body up to 30 centimetres thick (Image: Benita Chambers)

New fossils found in Australia tell us that enormous snakes evolved from predatory lizards like goannas, scientists say.

Palaeontologist Dr John Scanlon describes the well-preserved 25 to 20 million-year old snake skulls from the Riversleigh World Heritage site, in today's issue of the journal Nature.

"There's been quite a bit of controversy about what sort of lizards snakes evolved from," says Scanlon, of the Riversleigh Fossils Centre in Mount Isa.

He says one idea is that snakes evolved from small burrowing insect-eating lizards that lost their legs and developed an elongated body.

Another idea is that snakes evolved from relatively large predatory lizards such as goannas.

Scanlon says he hopes the skulls, from a now extinct snake known as Yurlunggur, will settle the debate on the evolution of snakes.

The structure of the jaws and face prove that Yurlunggur, an Arnhem Land Aboriginal term for 'rainbow serpent', is a very primitive snake, says Scanlon.

"It has so many features in common with lizards rather than other modern snakes."

For example, Yurlunggur has a clear jugal bone, similar to a cheekbone, not present in modern snakes.

And, in particular, the skulls show the snake is more closely related to large predatory lizards than to smaller insectivorous ones, says Scanlon.

"It basically shows a relationship between the most primitive snakes and the varanoid lizards such as goannas and mosasaurs," he says, referring to the giant acquatic goannas that lived during the Cretaceous period.

Two different views of one of the skulls (Image: John Scanlon)

The Yurlunggur skulls together form nearly a complete skull and mandible.

Along with numerous fossilised vertebrae found previously in Riversleigh, the skulls suggest Yurlunggur would have been 6 metres long and 20 to 30 centimetres thick.

Yurlunggur belongs to a group of snakes called madtsoiids, which include the second fossil snake to be described in Australia, Wonambi naracoortensis.

Scanlon hopes the most recent find will convince some scientists who argue madtsoiids are modern, not primitive, and therefore don't tell us about the early history of snakes.

"I'm hoping that this beautifully preserved skull will be more persuasive evidence than we've had before," he says.

A rare find

The discovery of such complete snake skulls is unusual, says Scanlon.

"Snake skulls are very flexible because they are specialised in swallowing things bigger than their own heads," he says.

"Compared to crocodiles and mammals, which have really solidly constructed skulls, snake skulls tend to just fall apart quite soon after death. It's very rare to find complete ones."

Scanlon says the Riversleigh fossil deposits are unique. While much more primitive snake fossils from the Cretaceous period have been found elsewhere in the world, these have been crushed flat.

But the Riversleigh deposits are in soft freshwater limestone that hasn't been compressed.

"The great thing about these Riversleigh bones is you can get them out of the rock using acid [to dissolve the limestone] and they are fully three-dimensional."